Thermal enhanced oil recovery (EOR) is a class of techniques well-known to those skilled in the art for increasing the oil production rate and ultimate oil recovery fraction in oil drilling processes. Thermal EOR has been particularly applied to oil-bearing formations where the oil has low mobility, whether due to characteristics of the formation (low permeability), or to characteristics of the oil (high viscosity), or both.
Several steam injection processes are well-known in the art, including the use of vertical and horizontal injection wells, and the use of continuous or intermittent steam injection. In some cases, steam is co-injected with gases, surfactants, solvents, or other substances that change the physical and chemical properties of the steam and the oil in the formation. Well-known techniques include SAGD (steam-assisted gravity drainage), SAGOGD (steam-assisted gas-oil gravity drainage), CSS (cyclic steam stimulation), steamflood, and steam drive.
The cost of steam represents a significant fraction of the total cost of oil production in thermal EOR operations. Accordingly, efficiently and cost-effectively supplying steam is a subject of key importance to the economics of oil production operations. Existing advances in the design and construction of steam generators, and systems for cost-efficiently treating feedwater, include reusing water produced from the oilfield and controlling the distribution and injection of steam across a field comprising multiple concurrent or sequentially operating injection wells. FIG. 1A schematically illustrates a conventional steam generation and distribution arrangement in which one or more centralized steam generation facilities deliver steam to a plurality of injection wells. In this arrangement, one or more steam generators feed a plurality of steam injection wells through a distribution network. The flow to each injection well can be apportioned in a manner that distributes stream relatively uniformly across the injection wells, reducing the effect of pressure drops in the distribution network and variations in subsurface pressure or injectivity. Accordingly, the system can include a flow rate control device in the steam flow path to each well, which is typically installed either at the wellhead or at a distribution manifold. Such rate control devices can include a “choke” which establishes critical flow conditions at the desired steam injection rate. Such flow control devices provide a low-cost way to establish a roughly uniform allocation of steam from the network to each of a plurality of injection wells. Some installations use motor-operated valves. For example, such valves are used in “cyclic steam stimulation” projects where the steam at an individual injector is turned on and off, typically multiple times per year, but typically less often than once per week or once per month.
In some instances, the use of solar steam generators can significantly reduce costs, emissions, and fuel use for oilfield steam generation. Unlike fuel-fired steam generators, solar steam generators generate steam at varying rates. Solar steam generators can deliver steam directly from concentrated solar energy collectors, or indirectly, after the solar energy has been transferred through an intermediate heat transfer fluid and/or heat storage device. Whether delivered directly or indirectly, however, solar energy is available in a time-varying manner. A representative solar steam generator is shown in FIG. 1B.
In some cases, fuel-fired steam generators are operated in concert with solar-powered generators to complement the time-varying output of the solar-powered generators. To maintain a relatively constant injection rate, fuel-fired steam generators are turned up or down in firing rate, based on the current availability of solar steam. Steam flow control devices at the wellheads play a role in balancing the flow rate into each well; these can be fixed choke or manually variable valves. FIG. 1C illustrates an example of solar steam generators intertied to a distribution network. FIG. 1D (taken from U.S. Pat. No. 8,701,773, assigned to the assignee of the present application) illustrates the respective contributions of fuel-fired steam (line 12) and solar steam (line 10) on an hourly basis, with the total steam injection rate roughly constant over the course of a 24-hour day (line 13), or greater during daylight hours (line 11).
In some instances, solar steam is more valuable than fuel-fired steam, e.g., due to lower cost, lower emissions, and/or other benefits. In such cases, systems and methods that enable solar steam to deliver a relatively greater fraction of the total annual injected steam volume are beneficial. The production and injection of steam at time-varying rates is one method which can enable solar steam to deliver a relatively greater fraction of total annual steam. For example U.S. Pat. No. 8,701,773 (referenced above) discloses the production of steam from the combination of both solar steam generators and fuel-fired steam generators at a time-varying total rate. However, improvements are needed in the systems and methods for dividing, distributing and injecting steam at time-varying rates to improve system efficiency.